Well stimulation involves the treatment of natural gas or oil wells via various methods to increase the productivity of the wells. One method used to stimulate wells is hydraulic fracturing. In hydraulic fracturing a fracturing fluid is pumped into the well at a higher pressure than the natural reservoir pressure. This causes the subterranean formation to crack, or fracture. If left unsupported the fracture would close, due to the surrounding formation pressure, once the higher pressure was removed. To prevent closure of the fracture, proppants are deposited in the fracture before the higher pressure is removed. Proppants are typically composed of materials such as sand, glass beads, polymers, walnut shells, porous ceramics, and dense ceramics.
In the past inorganic materials have often been utilized as a parting agent in the proppant formation process. The parting agents are used to enhance the separation of the green proppants after formation and to prevent them from agglomerating or sticking to the kiln during heat treatment. Generally, in these processes the inorganic material is not substantially different than the substrate material used to form the proppant, and in some cases are the same material (see, e.g., U.S. Pat. No. 4,427,068). However, there are some standard parting agents in use which are used irrespective of the proppant substrate material such as zircon, talc, silica, and alumina and are typically used in the amounts of 5 to 40 weight % of the dry proppant material (see, e.g., U.S. Pat. No. 4,680,230). The only consideration given to the selection of parting agents is that they do not interfere with the proppant formation and that they are unaffected by the heat treatment.
There are four major parameters associated with proppants and these are cost, density, strength, and heat resistance. All four parameters must be considered when choosing the best proppant to suit individual well properties. Typically, the highest strength and highest heat resistant proppants also have the highest density. The highest strength and temperature resistant proppants are made from bauxite, a fully dense ceramic, but unfortunately they also have the highest apparent density (˜3.6 g/cc) and high cost. Sand is attractive as proppant because of its low cost, however it has a lower strength and temperature resistance than bauxite and a relatively high density. (˜2.7 g/cc). Other proppants, such as polymer based proppants, walnut shells, and porous ceramics have low densities (˜1.08, 1.25, and 2.08 g/cc, respectively), but also have lower strengths and poor heat resistance.
Thus, there continues to be a long-felt need for a proppant which is lightweight, economical, high in strength, and highly temperature resistance.